Boolean Operators

Boolean Logic

An English mathematician by the name of George Boole developed a type of algebraic logic that is the basis of most computerized searching that we use today. It is used to search in databases, and it is also widely used by Internet search engines. A basic understanding of Boolean logic will help you to understand what is going on no matter where you seek electronic information.

Boolean logic involves understanding how sets of things overlap. The basic Boolean operators are:

and
or
not (sometimes written and not)

If you type the search statement CATS and DOGS you will retrieve all items containing both words, but no items containing just one of them. This is illustrated by the shaded portion of the diagram below.

You can see how this narrows down the number of items retrieved.

If you type the search statement CATS or DOGS you will retrieve any item that contains either word, even if it does not contain both. The shaded portion of the following diagram illustrates this relationship.

Obviously, this strategy greatly increases the number of items retrieved.

The third search operator is less often used. If you type CATS not DOGS, you will retrieve any item containing the word CATS, but only if it does not also include DOGS. This will eliminate some items containing both words that you may have wanted.

The Boolean operators are allowed in nearly all proprietary databases such as WilsonWeb, InfoTrac, PsycLit, etc. They are also used in some Internet search engines, but there is no uniformity about how and when they are allowed in search engines (some don't permit them at all), so it is important to read searching tips that are offered on the search engine homepage to find out what Boolean operators are used.

Although the most basic use of Boolean operators is illustrated above, they can be used in more sophisticated ways under some circumstances once you become familiar with them. For example, they can be grouped using parentheses just as you would in a mathematical equation.

(CAT or FELINE) and (DOG or CANINE)

Just as in mathematics, the operators within the parentheses will be executed first, and then the one outside. You would retrieve all items containing either of the two words within the first set of parentheses, then those containing either of the two words in the second set of parentheses, and then narrow the retrieval down to only those items that contain at least one word from within both sets of parentheses. You can see how this technique can be used to write very complicated search statements, which often aren't terribly effective. But the operators are very useful in narrowing down a search when used properly.

Less Common Search Operators

A few databases and search engines employ other words called Proximity Operators. These will include words such as:

near - CATS near DOGS usually means they must be in the same sentence

within - CATS within 3 DOGS usually means they must be within three words of each other

in - CATS in TI usually means that the word cats must be in the title of the piece

These can be very useful in fine-tuning a search, but are not supported in all databases and search engines. There are others, so always check the Search Tips offered by the search engine or database.